A typical printed circuit board has a copper conductor pattern on an insulating support. Tin of solder is applied onto the copper substrate, typically by electroplating. A standard solder film is nominally 0.0003 inches thick and a standard tin film has approximately the same or slightly less thickness. After the tin/solder film is applied to the board, a thin film of copper-tin alloy forms between the copper and the film. This thin film of copper-tin alloy is typically 0.000002 to 0.000004 inches thick. The copper-tin alloy thickness increases in thickness over time.
As used in the specification and claims, the word solder includes the various low melting point alloys and elements used for electrical soldered connections and for copper etching masks or resists. The majority of such coatings comprise various tin-lead alloys, but can also include alloys containing silver, bismuth, cadmium, indium, and other metals. Such films are produced using various methods, including chemical plating, chemical deposition, chemical displacement and immersion in a melt.
In the manufacturing process, the tin or solder film is stripped from the copper substrate. Generally, two types of compositions have been generally used in the past for tin and solder stripping. The most widely used prior compositions were based on an acid solution containing hydrogen peroxide and fluoride. In recent years, formulations based on nitric acid solutions containing ferric ions have become widely used commercially.
The prior art peroxide-fluoride solutions are undesirable because there is an exothermic reaction during stripping which heats the solution to a temperature which decomposes the unstable peroxide and makes the solution unusable. Hence, the solution requires cooling during use. Also, the peroxide-fluoride solutions are slower in operation than the nitric acid solutions.
Nitric acid solutions eliminate the problems associated with the peroxide-fluoride solutions. Early nitric acid based solutions comprised a two-solution system. The first acidic solution contained nitric acid to strip the tin or solder. The second acidic solution contained ferric chloride, ammonium persulfate, a mixture of sulfuric acid and hydrogen peroxide, or an acidic peroxide-fluoride mixture to dissolve the tin-copper alloy.
Further, the basic compositions and methods for single bath and spray nitric acid/ferric stripping are now well described in prior art patents. For example, the composition of U.S. Pat. No. 4,713,144 utilizes a combination of nitric, ferric, and sulfamic acid which quickly strips both the tin/solder film and the tin-copper alloy while leaving the copper surface bright, shiny and uniform.
Printed circuits in the telecommunications industry must have extremely high resistance insulating materials between the printed circuits to prevent the electrical signals from crossing over to adjoining circuits. The cross-over of electrical signals to adjoining circuits which can result from a low insulation resistance can cause, among other things, the ability to hear another person's phone conversation when you are using a phone.
Incomplete copper stripping or etching as well as the presence of a wide variety of metal species or other contaminants left on the printed circuit board after tin or solder stripping can result in a relatively low insulation resistance. Current nitric acid/ferric solutions are not formulated to maximize the insulation resistance between printed circuit traces.
Accordingly, it is an object of the present invention to provide a new and improved stripping composition and method of stripping tin or solder which provides the printed circuit board with enhanced insulation resistance.
This and other objects, advantages, features and results of the present invention will more fully appear in the course of the following description.